The present disclosure relates to a fixing device to fix a toner image on a sheet and an image forming apparatus including the fixing device.
An image forming apparatus, such as a laser printer, is provided with a fixing device configured to fix a toner image on a sheet. The fixing device includes a fixing belt and a pressing roller. The fixing belt is wound around a fixing roller, and the pressing roller is pressed against the fixing roller via the fixing belt to form a pressing area between the pressing roller and the fixing belt. When the sheet is passed through the pressing area, the toner image is fixed on the sheet.
By the way, in the image forming apparatus, when a thick paper or a rough paper is passed through the pressing area, a conveyance speed of the sheet may be set to be slower than a reference conveyance speed in order to keep a fixing performance constant. On the other hand, even when a plain paper is passed through the pressing area, the conveyance speed of the sheet may be set to be slower than the reference conveyance speed in order to increase resolution of the image or to reduce noise. In this case, different from the former case, because a sufficient fixing performance is provided at the reference conveyance speed, it is required to change a fixing condition. For example, a fixing temperature may be decreased or a fixing pressure may be decreased to shorten a length of the pressing area along the conveyance direction and to shorten a time (a passing time) required for the sheet to pass through the pressing area.
However, in the case where the fixing temperature is decreased, a waiting time until the fixing temperature is decreased sufficiently is generated, and a productivity of the fixing operation may be lowered. Additionally, because the time required for the sheet to pass through the pressing area becomes longer than that at the reference conveyance time, glossiness of the image tends to be high under a condition where the fixing performance suitable for the reference conveyance speed is provided, and an image quality may be unstable.
On the other hand, in the case where the fixing pressure is decreased to shorten the length of the pressing area and to shorten the passing time, the following problem may occur. It will be described with reference to FIG. 4. FIG. 4 is a graph showing a relationship between the fixing pressure, and the length of the pressing area and a linear pressure in the pressing area. In the graph, the horizontal axis shows the fixing pressure (N), the left side vertical axis shows the length of the pressing area (mm) and the right side vertical axis shows the linear pressure (N/mm) in the pressing area. The linear pressure is obtained by dividing the fixing pressure by the length of the pressing area.
At the reference conveyance speed, when it is assumed that the fixing pressure is 200 N and the length of the pressing area is 8 mm, the linear pressure is 25 N/mm. When the conveyance speed is decreased to one half of the reference conveyance speed, in order to keep the passing time constant, it is required to set the length of the pressing area to 4 mm. Then, the fixing pressure is about 50 N and the linear pressure is about 12.5 N/mm which is one half of that at the reference conveyance speed. When the fixing pressure is decreased such that the passing time is the same as that at the reference conveyance speed, the linear pressure is also decreased. When the linear pressure is low, the glossiness of the image tends to be lower. Therefore, it is difficult to provide the same fixing performance as that at the reference conveyance speed because the stable image quality cannot be obtained.
When the conveyance speed is decreased, the fixing pressure may be adjusted so as to make the passing time slightly longer than that at the reference conveyance speed and to make the linear pressure slightly lower and the fixing temperature may be adjusted. However, it is difficult to provide the same fixing condition as that at the reference conveyance speed and to solve the above problems regarding the image quality and the productivity.
In some cases, the fixing device capable of changing the pressure of the pressing area may be provided with a soft or hard pad which presses the pressing belt to the heating roller or a pressing member which presses the fixing belt toward the sheet conveying path. Alternatively, a movable pressing part which comes into pressure contacts with the fixing belt or a nip shape changing part which changes the tension of the fixing belt may be provided.
However, the above fixing devices are configured to change the pressure of the pressing area depending on the type of the sheet or the desirable glossiness; and are not applied to change the pressure of the pressing area depending on the decreased conveyance speed when the plain paper is passed through the pressing area.